1. Field of the Invention
This invention relates to the use of a particular thixotropic cement for sealing or cementing subterranean zones penetrated by a borehole, such as cementing the annular space between an oil or gas well casing and the surrounding formation, where said compositions establish sufficient static gel strength in a time frame sufficiently short to preclude gas migration or fluid flow through said composition prior to the time that the composition sets to a hardened state.
2. Description of the Prior Art
It is common practice in operations conducted to produce hydrocarbons from subterranean formations, to cement or seal the area between the drill pipe and the formation wall. This is accomplished via either directly introducing the cement into the space between the formation wall and the outer side of the casing or via pumping the cement into the casing with sufficient pressure to force the cement back up the annular space between the outside of the casing and the formation wall.
The zones adjacent the cement-containing annulus can contain fluids or gas ("formation fluids") under pressure. Often these formation fluids can enter and flow through the cement-containing annulus. The most common problem relates to annular gas flow (also called gas leakage or gas migration), which refers to the flow or migration of gas in a cemented casing-borehole annulus. Such gas can flow back to the surface, create communication between producing or other subterranean zones and can, when in high enough volume, create blowouts between the period of placement and before actual set. Minor interzone gas flow problems can sometimes be tolerated, although often at the expense of lower production. When the magnitude of leakage requires remedial action, expensive squeeze cementing jobs are necessary to control such interzone gas flow.
The theory of how formation fluid flow occurs revolves around two key cement slurry parameters, static gel strength and volume reduction. These parameters affect the cement column's ability to transmit hydrostatic pressure.
The first parameter, static gel strength, is the development of some internal rigidity in the matrix of the cement that will resist a force placed upon it. The development of static gel strength will start to occur immediately after pumping has stopped and will continue to increase until the cement has set. At some time before actual set, the cement will develop a static gel strength high enough to prevent any fluid from moving through it. Test have indicated that a gel strength 500 lbs/100 ft.sup.2 is sufficient to prevent any movement, although at certain conditions such gel strength can be considerably lower. When the cement has developed a static gel strength high enough to prevent formation fluids from moving through it, the cement is said to have completed its transition phase. The cement column can now begin to support some of its own weight.
Volume reduction can occur in two ways. Fluids can be lost from the matrix of the cement slurry to the formation. Even when fluid loss values are very low, small amounts of fluid are still lost from the slurry and can result in a large pressure drop. Additionally, as the cement hydrates there is a hydration volume reduction. Such reduction can ultimately be as high as 3 percent. Where the static gel strength development and the volume reduction are sufficient for a pressure drop to be realized, the hydrostatic pressure can fall below the formation pressure and formation fluids can enter the cement filled annulus. If the gel strength of the cement slurry is not high enough to prevent further movement of formation fluids, a fingering or migration phenomenon will occur. However, where the gel strength is high enough and the formation fluids are not already moving through the cement column, flow will not be initiated. Traditionally, the petroleum industry has attempted to prevent formation fluid flow by increasing the slurry density, improving mud displacement, controlling mud-cement slurry compatibility, using fluid loss control additives, and multiple stage cementing. Although these techniques are helpful and have shown some measure of success, none have completely solved formation fluid flow problems.
U.S. Pat. No. 3,959,003 and 3,804,174 describe a cement composition that includes as an additive a complex reaction product of a water-soluble carboxyalkyl, hydroxyalkyl or mixed carboxyalkylhydroxyalkyl ether of cellulose and a polyvalent metal salt. The composition exhibits thixotropic properties and the preferred reaction product uses hydroxyethylcellulose and zirconyl chloride. Titanium complexes are not mentioned in these patents. Further, new testing procedures have indicated that the compositions of these patents exhibit rather limited thixotropic properties and in event do not exhibit sufficient thixotropic properties to meet the criteria for prevention of formation fluid migration of the present invention.
New techniques using cement slurries containing a stabilized, dispersed gas or cement slurries capable of internally generating gas have achieved a much greater degree of success. Such techniques are described in U.S. Pat. Nos. 4,304,298 and 4,340,427. However, there may be cases where it is undesirable to use such compressible cements, be it from logistical, economic, time or other standpoints.
Accordingly, a need exists for a method of cementing oil and gas wells that provides a highly thixotropic cement slurry that develops high gel strengths in a sufficiently short time span so as to prevent fluid invasion of the annulus even though the hydrostatic pressure may fall below the gas reservoir pressure during the transition of the slurry to a solid mass.